Introduction Smoke-free policies are often regarded as an imposition on the economy which has hindered their adoption in countries like China. In reality, they increase workforce productivity, including via fewer sick days (absenteeism) and unsanctioned breaks (presenteeism), culminating in an increase in effective work hours. This paper measures the potential impact on productivity levels in China via these short-term channels following the introduction of a national smoke-free law for indoor public places. Methods We use existing systematic reviews to identify effect sizes for smoke-free policies on smoking behaviour in the workplace, with this evidence being supplemented by local studies. A baseline model of aggregate hours worked across China’s economy was developed using a 2018 survey of adult smoking, National Account Statistics and International Labour Organization data. The effect sizes were introduced to test the impact of the law on effective hours worked by those who quit smoking. Results A national smoke-free law would cause the smoking rate among employed adults to decrease by 5.3% in relative terms, representing 11.6 million fewer adults who smoke. This decrease in smoking would, in turn, increase total effective work hours across China by 0.07% and expand Gross Domestic Product by ¥90 billion per annum or by 0.10% on current levels. Conclusion The productivity gains from a national smoke-free law in China are potentially very sizeable even before considering the impact of long-term reductions in mortality and healthcare spending. As China’s working-age population declines, such a law would strongly support high-quality development.